Structural acrylic adhesive systems typically are two-part systems composed of an activator and an adhesive, each applied to a separate substrate. On assembly, the activator and adhesive mix to generate radicals that in turn cure the acrylates or methacrylates present in the system. These acrylate or methacrylate monomers may contain a urethane backbone to impart increased flexibility and adhesion.
Typically, the activator will contain an amine reductant, such as a butyraldehyde/aniline adduct or a dihydropyridine. The adhesive will usually contain a peroxide as the oxidant. Such systems will give an adhesive fixing time of about one minute.
There has always been a need to increase the cure speed of these acrylic adhesives to achieve faster fixing times. One such adhesive is described, for example, in U.S. Pat. No. 4,432,829, issued 21 Feb. 1984 to Bachmann. This reference discloses an adhesive composition comprising, preferably, an activator part and an acrylic adhesive and a catalyst system part. The catalyst system comprises an aromatic perester free-radical precursor, maleic acid, and a transition metal cure accelerator to provide metal ions in an oxidized valence state. The activator is an amine/aldehyde adduct.
As is recognized in this reference, a difficulty commonly encountered in attempts to produce fast-acting adhesives is the tendency of the composition to cure prematurely. This is believed to be the reason the compositions in this reference utilize a perester free-radical precursor, and specifically avoid the use of hydroperoxides. The perester is more stable in the presence of transition transition metal ions, and consequently any premature gelling or curing is allegedly minimized. A further consequence of using the perester and not the hydroperoxide, however, is the loss of some rapidity of cure.
Thus, there is still a demand for adhesive formulations capable of rapidly developing high levels of adhesion, and particularly for those that do not cure prematurely.